Fix noIndex being enabled on production builds

[MattiasBuelens]

Summary

Production builds were silently marked noIndex, which makes @docusaurus/plugin-sitemap skip writing sitemap.xml entirely (and adds <meta name="robots" content="noindex, nofollow"> to every page — currently live on https://optiview.dolby.com/docs/).

Cause: the workflow passes docusaurus_no_index: 0 for production, which reaches the config as the string "0", and !!'0' === true.

-  noIndex: !!process.env.DOCUSAURUS_NO_INDEX,
+  noIndex: NO_INDEX,   // ['1', 'true'].includes(DOCUSAURUS_NO_INDEX.trim().toLowerCase())

PR previews (docusaurus_no_index: 1) keep noIndex: true.

Verified with a local DOCUSAURUS_NO_INDEX=0 npm run build: build/sitemap.xml is now generated and no page contains a noindex robots tag.

Link to Devin session: https://dolby.devinenterprise.com/sessions/b3059def9f1642a3bc87a647c751cdce
Requested by: @MattiasBuelens

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